The present invention relates to a bonding method and bonding device of substrates and a manufacturing method of a liquid crystal display device, and more particularly to a bonding method and bonding device for bonding a pair of substrates together by placing the substrates one upon another and sandwiching the substrates between a pair of mutually facing surface plates, and a method of manufacturing a liquid crystal display device by introducing liquid crystals into the space between a pair of substrates after bonding the substrates.
Conventionally, a substrate bonding method for bonding a pair of substrates together by placing the substrates one upon another and sandwiching the substrates between a pair of mutually facing surface plates has been used for the manufacture of various products.
For instance, according to a typical manufacturing method of a liquid crystal display device, a wiring layer, pixel electrodes, active elements and color filter are suitably formed on the inner surface of each of two pieces of glass substrates and coated with an alignment film or the like to provide two pieces of composite substrates. Next, these composite substrates are bonded together with a seal material therebetween by sandwiching them between a pair of mutually facing surface plates. Finally, liquid crystals are introduced between the two pieces of composite substrates to provide a liquid crystal display device.
For example, Tokukaihei No. 11-95181 (Japanese laid-open patent publication; published on Apr. 9, 1999) specifically explains a substrate bonding method for use in a manufacturing method of a liquid crystal display device. More specifically, as shown in FIG. 10 (corresponding to FIG. 1 of the above publication), in the step of bonding an element substrate 120 and a counter substrate 130 together, the counter substrate 130 is pressed from an upward direction by a bonding head 115 so as to stick to the element substrate 120 with a seal material (not shown) therebetween.
A rigid base plate 113 is positioned under the element substrate 120, and provided with a recessed section 113a formed at the center part thereof. An outer edge of the recessed section 113a is located in the vicinity of an outer edge section of a liquid crystal display area A of the element substrate 120 and counter substrate 130 and on a slightly inner side of the formation area of the seal material (not shown) More precisely, the outer edge of the recessed section 113a is located slightly outside of the outer edge section of the liquid crystal display area A.
A buffer material 114 is placed on the rigid base plate 113. The buffer material 114 is provided with a pierced section 114b having the same shape as the recessed section 113a. The recessed section 113a and pierced section 114b prevent a portion of the outer surface section of the element substrate 120, which is slightly larger than the liquid crystal display area A, from coming into contact with the base plate 113 and buffer material 114. Therefore, even if broken pieces and dusts are present between the element substrate 120 and the base plate 113, it is possible to prevent the outer surface of the element substrate 120 in the liquid crystal display area A from been scratched.
Besides, there is a method in which a non-contact section is provided by forming a pierced section 117b (or recessed section) in the buffer material 117 as shown in FIG. 11 (corresponding to FIG. 3 of Japanese laid-open patent publication (Tokukaihei) No. 11-95181) instead of forming the recessed section 113a in the base plate 113.
There is another method in which a recessed section 116b is formed at the center part of a press section 116a of a bonding head 116 as shown in FIG. 11 so as to prevent the outer surface of the counter substrate 130 in the liquid crystal display area A from being scratched. The outer edge of the recessed section 116b is located in the vicinity of the outer edge section of the liquid crystal display area A and on a slightly inner side of the formation area of the seal material (not shown).
However, the above-mentioned substrate bonding methods suffer from the following problems.
First, in either the method using the rigid base plate 113 provided with the recessed section 113b as shown in FIG. 10 (or the base plate 113 provided with the non-contact section) or the method in which the non-contact section is produced by forming the pierced section 114b (or recessed section) in the buffer material 114 as shown in FIG. 11, since the buffer material 114 positioned between the rigid base plate 113 and the element substrate 120 is not secured to either the base plate 113 or the element substrate 120, it tends to be displaced. For this reason, displacement often occurs between the base plate 113 and the buffer material 114 and between the buffer material 114 and the element substrate 120, and thus making it difficult to accurately align the element substrate 120 on the base plate 113. As a result, the distance (gap) between the substrates 120 and 130 is irregular and the alignment accuracy of the substrates 120 and 130 deteriorates. In actual fact, for such non-uniformity and instability of the distance between the substrates 120 and 130 and deterioration of the alignment accuracy, it is extremely difficult to mass-produce a liquid crystal display device while accurately keeping a slight and uniform distance (a cell gap of around a few xcexcm) between the substrates 120 and 130 over the entire surface of the substrates 120 and 130 in a stable manner.
Moreover, since the buffer material 114 is formed of a pile of thick paper, it has a low strength and low reliability (durability) fore repeated use. For this reason, when such a buffer material 114 formed of such a pile of thick paper is used repeatedly to bond the substrates for the mass-production of a liquid crystal display device, a problem arises. In actual fact, it is impossible to use this buffer material 114 in this manner. Furthermore, since the buffer material 114 is formed of a pile of thick paper, even if it is made stick to the base plate 113 by vacuum suction, it is difficult to completely secure the buffer material 114 to the base plate 113, and the buffer material 114 tends to be displaced. This causes non-uniformity of the gap between the substrates 120 and 130 and deterioration of the alignment accuracy of the substrates 120 and 130.
Additionally, a method using the rigid base plate 113 provided with the recessed section 113b as shown in FIG. 10 (or the base plate 113 provided with the non-contact section) suffers from the following problem. Specifically, whenever the dimensions of a liquid crystal display device to be manufactured are changed, it is necessary to replace not only the buffer material 114 with the one having a pierced section 114b corresponding to the changed dimensions of the liquid crystal display device, but also the base plate 113 with the one having a recessed section 113b corresponding to the changed dimensions of the liquid crystal display device. For this reason, this method can not readily meet liquid crystal display devices of a variety of dimensions.
Besides, the above-mentioned publication discloses a modified example of the structure shown in FIG. 11, in which a buffer material having a recessed section is made stick to a press surface 116a of a bonding head 116 having a recessed section 116b. However, this publication does not mention sticking the buffer material having a recessed section to a flat press surface 116a. Therefore, whenever the dimensions of a liquid crystal display device to be manufactured are changed, it is also necessary to replace the bonding head 116 with the one having a recessed section 116b corresponding to the dimensions of the liquid crystal display device. For this reason, this method can not readily meet liquid crystal display devices of a variety of dimensions.
An object of the present invention is to provide a substrate bonding method that can readily meet the manufacture of bonded substrates of a variety of dimensions.
In order to achieve the above object, a substrate bonding method of the present invention includes:
placing a pair of masks, which were prepared by forming a recessed section at a center part of each flat plate, on a pair of mutually facing surface plates so that the recessed sections of the masks face each other; and
sandwiching the pair of substrates positioned one upon another between the surface plates with the masks therebetween so as to bond the substrates together.
Moreover, the substrate bonding method of the present invention may place a mask, which was prepared by forming a recessed section at a center part of a flat plate and a protruding section in the recessed section, on at least one of the pair of mutually facing surface plates so that the recessed section of the mask faces the other surface plate, and sandwich the pair of substrates positioned one upon another between the surface plates with the mask therebetween so as to bond the substrates together.
Furthermore, the substrate bonding method of the present invention may place a mask, which was prepared by forming a recessed section at a center part of a flat plate and through-holes at parts other than the recessed section, on at least one of the pair of mutually facing surface plates so that the recessed section of the mask faces the other surface plate, stick and secure the substrate to the surface plate through the through-holes in the mask by performing vacuum suction from substrate attracting holes formed in the surface plate, and sandwich the pair of substrates positioned one upon another between the surface plates with the mask therebetween so as to bond the substrates together.
According to the above substrate bonding method, since the mask is provided with the recessed section, the surface of the center part of the substrate facing the recessed section of the mask is not in contact with the mask. Therefore, in a region where the recessed section of the mask and the substrate face each other, even if there are broken pieces of the substrate, foreign matter, etc. between the mask and the substrate, the broken pieces of the substrate, foreign matter, etc. come into the recessed section of the mask. Consequently, it is possible to prevent a local pressure due to the broken pieces of the substrate, foreign matter, etc. from being applied to the outer surface of the substrate facing the recessed section of the mask. It is thus possible to prevent the outer surface of the substrate facing the recessed section of the mask from being scratched. In other words, it is possible to protect the outer surface of the substrate from foreign matter, etc. by the recessed section of the mask.
Additionally, in the manufacture of a liquid crystal display device using the bonded substrates, in general, spacers formed of plastic beads are provided on the inner side of the substrate. However, if a local pressure is applied to the outer surface of the substrate, the broken pieces of the substrate, foreign matter, etc. press the substrate against the spacers, and thus the inner surface of the substrate is scratched. However, according to the above-mentioned method, since a local pressure is not applied to the outer surface of the substrate facing the recessed section of the mask, it is also possible to prevent scratches on the inner surface of the substrate in this section.
Therefore, for example, if a liquid crystal display device is manufactured using a portion of the bonded substrates, which is protected by the recessed section of the mask, it is possible to provide a liquid crystal display device having no scratches in a display screen (display area).
Moreover, according to the above-mentioned method, since the contact area of the mask and the substrate surface is reduced, it is possible to decrease the amount of foreign matter, etc. sandwiched between the mask and the substrate.
Furthermore, according to the above-mentioned method, in a region where the substrate and the recessed section of the mask face each other, since foreign matter, etc. are not sandwiched between the outer surface of the substrate and a press surface, the distance (gap) between the substrates can never be decreased locally due to the effects of the sandwiched foreign matter, etc. It is thus possible to keep the distance between the substrates uniform and provide bonded substrates with a uniform gap.
Besides, according to the above-mentioned method, since a pair of masks are placed on the surface plates, respectively, it is possible to prevent both of the substrate surfaces from being scratched and both of the surface plates from being scratched by foreign matter, etc. Additionally, since the dimensions of the recessed section can be changed easily by replacing the mask, this method can readily meet bonded substrates of a variety of dimensions, such as a liquid crystal display device having a liquid crystal display area of a variety of dimensions.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.